Result is +Number
which should be preferred for portability.
Number and Result have to be of the same type.
In coroutining mode, if Number is uninstantiated, the call to +/2 is delayed until this variable is instantiated.
Success:
+(1, 1).
+(5, Result). (gives Result = 5)
+(-6.2, Result). (gives Result = -6.2)
Fail:
+(1, 0).
+(1.0, 1).
+(1, 1.0).
+(1, r).
Error:
+(A, 6). (Error 4).
+(4 + 2, 6). (Error 24).